Oscillographic writing system



y 5, 1970 CUE. JOHNSON, JR 3,510,878

QSCILLOGRAPHIC WRITING SYSTEM Filed April 2, 1968 3 Sheets-Sheet 1 Z 8 1 4 l6 12 f 4 6 I HIGH PRESSURE FEED VERTICAL HORIZONTAL FERROFLUID VALVE DEFLECTION DEFLECTION TARGET SUPPLY UNIT UNIT I I L K //0"TVX:)

J INPUT ,4 GAS l I- DATA 106 Ls Pfg' SIGNALS CZARK E JOHNSON, Jr.

INVENTOR.

ATTORNEY y 5, 1970 c. E. JOHNSON, JR 3,510,878

OSCILLOGRAPHIC wai'rme SYSTEM Filed April 2, 1968 3 Sheets-Sheet 2 EIMPRESSDR 44 50 I 42 40 r 38 VERTICAL FERRDFLUID FEED AND HORIZONTAL I sUPPLY vALvE DEFLECTON 46 T I 52 J I UNITS I g 34 I STEPPING 55 MoToR 54 F 4 DRIvE INPUT 5 DAT DATA PROCESSOR ANTENNA 0 62 4 f l DATA SIGNAL RFAMPLIFIER DEMDDULATCR AMPLIFIER 68 L CURRENT I AMPLIFIER X 66 CURRENT DATA AMPLIFIER 1y TO DEFLECTIoN UNITs, SIGNAL 68 r FEED vALvE AND sEPARAToR CURRENT STEPPING MOTOR AMPLIFIER 1F 66 68 CURRENT I 1 AMPLIFIER CZARK E. JOHNSON, L/r.

. INVENTOR.

ATTORNEY May 5, 1970 Filed April 2, 1968 C. E. JOHNSON, JR

OSCILLOGRAPHIC WRITING SYSTEM 3 Sheets-Sheet 5 74'\ INPUT TRANSDUCER CURRENT x AMPLIFIER DEFLECTIoN UNIT 82 84 k I T 78 "oR" CURRENT FEED 55 E GATE AMPLIFIER vALvE r72 [80 INPUT TRANsDUCER CURRENT Y Y AMPLIFIER DEFLECTIoN UNIT X 90 INPUT DIFFERENTIATDR 94 96 f IOO v SQUARING fi AM I PL'FER SUMMING gggfi SQUARING AMPUHER GENERATOR AMPLIFIER I I I AMPLITUDE TO i I/JLFREQUENCY coNvERTER Y PROPORTIONING D FF R IA /-92 II INPUT E ENT TOR FEED vALvE INVENTOR.

ATTORNEY United States Patent Office 3,510,878 OSCILLOGRAPHIC WRITING SYSTEM Clark E. Johnson, Jr., Weston, Mass., assignor to Vibrac Corporation, Chelmsford, Mass., a corporation of Massachusetts Filed Apr. 2, 1968, Ser. No. 718,167 Int. Cl. G01d 9/40 U.S. Cl. 346-1 12 Claims ABSTRACT OF THE DISCLOSURE An oscillographic writing system employing a stream of ferrofluid ink directed at a writing surface and means providing a gradient magnetic field for deflecting the stream to produce a predetermined writing pattern.

This invention relates to a new and improved oscillographic writing system involving the use of ferrofluids.

PRIOR ART A ferrofluid is a ferromagnetic fluid with a magnetic polarizability that is substantially uniform and having the property such that when a magnetic field is applied thereto, a body force is developed within it which is suflicient to change radically its gross behavior without altering its fluid characteristics. It is unlike a conventional magnetic clutch fluid of the type consisting of ferrite particles which chain together and solidify under applied fields. At least two types of ferrofluids are known. One type consists of a colloidal suspension of submicron-sized ferrite particles in a carrier fluid such as kerosene, with a dispersing agent added to prevent flocculation. This type of ferrofluid is described by R. E. Rosensweig, Magnetic Fluids, International Science and Technology, July 1966, pp. 48-56. Another type consists of liquid solutions of magnetic polymers. This type is exemplified by U.S. Pats. 2,378,441 and 3,282,909. The present invention takes advantage of the facts that a ferrofluid retains its fluidity in a magnetic field and a mass of ferrofluid can be made to move by subjecting it to a gradient magnetic field.

THE INVENTION The primary object of the present invention is to provide a new method of writing using a ferrofluid ink.

A specific object of the invention is to provide an oscillographic writing system wherein graphic displays are formed by means of a ferrofluid ink.

A further object of the invention is to provide a method of writing with a ferrofluid ink wherein the written image is determined by magnetic deflection units controlled by electrical signals.

Described briefly the invention involves selectively establishing a jet of a ferrofluid ink directed at a writing surface and deflecting the jet by means of a gradient magnetic field so as to cause the jet to undergo excursions according to a predetermined writing pattern.

Further details and other objects and advantages of the invention are set forth or rendered obvious to persons skilled in the art by the following detailed specification which is to be considered together with the accompanying drawings wherein:

FIG. 1 is a schematic diagram showing the basic components of an oscillographic ferrofluid writing system made in accordance with the present invention;

FIG. 2 is an end view looking from left to right in 3,510,878 Patented May 5, 1970 FIG. 1 of a pair of pole pieces for the vertical deflection unit;

FIG. 2A is a side view of the pole pieces of FIG. 2;

FIG. 3 is a schematic diagram of a preferred embodiment of the invention;

FIG. 4 is a schematic diagram of a receiver for receiving and demodulating a radio frequency carrier signal to recover data signals for use in the embodiment of FIG. 3;

FIG. 5 illustrates a modification of the invention where the horizontal and vertical deflection signals derived from separate transducers are employed to control the flow of ferrofluid ink; and

FIG. 6 illustrates an arrangement for providing a constant deposit of ink per unit length of travel on the target medium.

Turning now to FIG. 1 an oscillographic writing system of the present invention basically comprises a source of supply 2 of ferrofluid ink under suitable pressure, a delivery tube 4 which is connected at one end to the source of ink supply 2 and at the other end has a nozzle 6 adapted to discharge the ferrofluid ink as a high velocity jet stream of relatively small cross-section. Interposed in delivery tube 4 is an electrically controllable feed valve 8 which is adapted to control the flow of ink to the nozzle 6. The feed valve may take various forms, but preferably it is a conventional solenoid-controlled valve of a type having relatively fast response so as to permit the jet stream to be turned on and off rapidly. The nozzle 6 is disposed so that the jet stream is directed toward a target 10 having a writing surface 12 adapted to accept ink impinging thereon and to hold said ink with a minimum of diffusion and dispersion so that little ink is required to write an image and the writing is relatively sharp. By way of example but not limitation, the target 10 may be made of paper. The system further includes a vertical deflection unit 14 and a horizontal deflection unit 16. The order of these units is a matter of choice, but preferably they are located as closely as possible to one another without interaction of their magnetic fields. The deflecting units are interposed between nozzle 6 and target 10 and provide gradient magnetic fields in positions to effect horizontal and vertical deflections of the jet stream of ferrofluid ink. The valve 8 and deflection units 14 and 16 are controlled by input data signals corresponding to the information signals employed to drive a cathode ray oscilloscope. The input data signals may be derived from various sources obvious to persons skilled in the art, notably from analog transducers (e.g. transducers measuring dynamic process parameters such as temperatures, pressure and rate of flow), computers, radio receivers, keyboard controlled signal generators, magnetic tape or punch card readout units, etc. Depending upon the input signals, the writing produced on the target may be in the form of alphabetical or numerical characters, graphs, pictures, etc.

As indicated above deflection of the jet stream is accomplished by deflecting the jet with gradient magnetic fields generated by the deflection units 14 and 16. Turning now to FIGS. 2 and 2A the vertical deflection unit consists of a pair of vertically aligned soft iron pole pieces 18 and 20. These pole pieces may be separate members, as shown, in which case each is provided with its own energizing coil as shown at 22 and 24. These coils are wound and energized so that the adjacent ends of the pole pieces constitute opposite, i.e., north and south, magnetic poles. The two coils may be connected so as to be energized by the same or separate electrical signals. If separate signals are employed to energize coils 22 and 24, they may be of the same or opposite polarity depending upon the manner in which the coils are wound. Preferably, however, the two pole pieces are integral portions of a single magnetic core. Most preferably, the single core is C- or U-shaped, and the pole pieces form the ends thereof. With such preferred construction, only one coil is required to be wound on the core to impart opposite magnetic polarization to the two pole pieces. Each of the two pole pieces is elongated parallel to the axis of nozzle 6 and is relatively narrow in a horizontal direction parallel to writing surface 12. The adjacent end face of pole piece 18 is flat but has a longitudinally extending rib 26 located in a vertical plane common to the axis of nozzle 6. The bottom surface 28 of rib 26 has a semicircular curvature. The adjacent end of pole piece 20 has a concave surface 30 which also is of circular curvature. The jet stream is represented by the circle 32 in FIG. 2.

With the arrangement shown in FIGS. 2 and 2A, when the pole pieces are magnetically energized in the manner above described, the lines of flux will tend to extend according to the minimum gap between the two pole pieces, with the result that they will converge at the surface 28 of rib 26. Hence the magnetic field between the pole pieces will have a distinct magnetic gradient, with the density of the field decreasing with increasing distance from rib 26 as represented schematically by the vector arrow shown in full lines in FIG. 2. Varying the amplitude of the currents flowing through coils 22 and 24 will cause the jetstream 32 to be deflected vertically in one direction or the other according to whether the change in current causes a decrease or increase in the strength of the gradient magnetic field.

It is to be noted also that the pole pieces may hie oriented so that the stream of ferrofluid ink flows transversely to the rib 26 that is, parallel rather than at a right angle to the plane of FIG. 2. The effect is similar since the magnetic field gradient is still vertical. However, the arrangement shown in FIG. 2 is preferred since it its easier to calculate the eifect of the gradient field.

The horizontal deflection unit is the same as the vertical deflection unit except that the pole pieces are rotated 90 from the positions shown in FIG. 2 so that the magnetic field gradient extends horizontally instead of vertically. Hence varying the current in the coils will cause the stream to be deflected horizontally. The degree of deflection by either deflection unit is a direct function of the magnetic field gradient in the X (horizontal) or Y (vertical) direction according to which unit is considered, the magnetic induction of the ferrofluid (which depends on the nature of the fluid and the overall magnetic field strength) and the transit time of the stream through the gap between the pole pieces. The degree of deflection is also an inverse function of the density of the ferrofluid. The transit time through the gap between the pole pieces depends upon the length of the pole pieces measured in a direction parallel to the axis of nozzle 6 and the velocity of the stream. The response time, i.e. the time required to deflect the stream, is related to the transit time in the gap and may be altered either by changing the field gradient or the stream velocity. By 'way of example, assume that the pole pieces of the vertical deflection unit both have a length of cm., the stream has a velocity of 3000 cm./sec., and the ferrofiuid has a density of approximately 1 and a magnetic induction at saturation of 1000 gauss. Under these conditions if the pole pieces are energized to provide a gradient magnetic field of 1000 gauss/cm., the stream will be deflected vertically about 2.8". If the distance between the vertical deflection unit and the writing surface 12 is about 25 cm., the amplitude of the vertical swing of the stream at the writing surface would be about 1.2 cm. Under these conditions the response time would be about 1.7 milliseconds. The same degree of deflection may be achieved but with a response time of about 0.5 millisecond by increasing the velocity to about 1000 cm./sec. and the gradient field to about 10,000 gauss/cm. The frequency of this system would be about 600 Hz. In this connection it is to 'be noted that although the frequency response is dependent upon stream velocity, it is affected far less by a change in stream velocity than is the amplitude of deflection. The latter varies approximately with the square of the change in stream velocity. Hence increasing the stream velocity by 3 decreases the deflection amplitude by a factor of 9.

It is believed to be obvious that the stream of fluid flows only when an input signal opens feed valve 8, and that in the normal course of events this will not occur except when input data signals are applied to one or both of the deflection units. However, depending upon the type of writing to be performed, valve 8 may be opened when no input data signals are applied to the deflection units, e.g., to plot points on a moving target such as a moving web of graph paper. The deflection units may be fully deenergized in the absence of input data signals, in which case the stream, if flowing, would assume (by virtue of the relative positions of nozzle 6 and the pole pieces of the deflection units) a predetermined reference spatial position with respect to the pole pieces, and applied data signals would deflect it back and forth to one side of said reference position. On the other hand, both deflection units may be continuously energized by a predetermined bias current, in which case the ferrofluid stream would assume a reference position determined by the magnitude of the bias current and would be deflected back and forth on either side of the deference position by applied data signals.

Turning now to FIG. 3, a preferred embodiment of the invention comprises a tank 34 containing a supply of ferrofluid ink maintained under pressure by a compressor 36. The tank is connected to a discharge nozzle 38 by way of delivery pipe 40 and a solenoid controlled valve 42. The nozzle is positioned to deliver a jet of ink through vertical and horizontal deflection units identified collectively at 44 to a writing surface in the form of a web of paper 46 which is fed from a supply roll 48 to a take-up or rewind roll 50. The two rolls are mechanically rotated by a conventional stepping motor drive system 52.

Coupled between an input terminal 54 and feed valve 42, deflector units 44 and stepping motor drive 52 is a data processor 56. Input data in the form of a complex signal wave made up of different signals for opening and closing the feed valve and operating the deflection units and the stepping motor drive is fed in at terminal 54. The data processor is adapted to separate the different signals and direct them to the valve, the deflection units and the stepping motor drive. In the case of writing a series of words, the deflection signals for successive characters are spaced timewise by an amount sufficient to permit the stepping motor drive to index the paper web,

while the signals to the feed valve and the stepping motor operate to allow flow of ink only during the time that writing signals are being applied to the deflection units and to advance the web of paper in the period between deflection signals for successive characters. By using only one deflection circuit (the one deflecting the ink stream transversely to the direction of movement of the web) and operating the stepping motor drive continuously, it is possible to plot a graph on the web. The writing signal for the single deflection unit may be derived from any suitable transducer.

The form and function of the data processor 56 depends on the form of the input data. By way of example but not limitation, the input data may be transmitted from a remote source in the form of an R.-F. carrier wave modulated with the necessary data signals, in which case the data processor would take the form of a receiver having means for detecting and amplifying the various signals. FIG. 4 shows a receiver suitable for processing input data in the system of FIG. 3.

Turning now to FIG. 4, a transmitted R.F. carrier wave modulated with data signals as above described is picked up by an antenna 58 and fed to an R.F. amplifier 60 for amplification. Thereafter it is fed to a conventional demodulator 62 which separates the carrier from the data signals and feeds the latter to a data signal amplifier 64. After amplification to a suitable level the data signals are fed to a data signal separator 66 which preferably consists of a plurality of bandpass filters each designed to pass a different one of the data signals and to reject the other data signals. Alternatively signal separator 66 may comprise a plurality of tuned amplifiers. The different data signals are then passed to separate current amplifiers '68. FIG. 4 shows four current amplifiers producing output data signal currents I I I and I The currents I and I constitute the signals for feed valve 42 and stepping motor drive 52 respectively. The currents I and I constitute the currents for the horizontal and vertical deflection units respectively. Application of the currents I and I to the corresponding deflection units produces gradient magnetic fields in the latter, with the gradient increasing with increasing current.

FIG. 5 illustrates how a ferrofluid writing system having a stationary writing surface may be operated directly from two input transducers of the type producing a small analog voltage proportional to the parameter being measured, e.g., temperature, pressure, acceleration, rate of flow, torque, etc. In this case two input transducers 70 and 72 are connected to separate current amplifiers 74 and 76 respectively which are adapted to produce output currents which exhibit relatively large amplitudes in response to relatively small input voltages, e.g., in the order of millivolts. The outputs of amplifiers 74 and 76 are fed to x and y deflection units 78 and 80 respectively and also to an or gate 82 adapted to produce an output signal whenever and so long as an input is received from either transducer or both transducers. The output of gate 82 is applied to a third current amplifier 84 which produces a relatively large current so long as it is receiving a signal from gate 82. The output of amplifier 84 is used to operate an ink feed valve 86. With this arrangement the input signals from transducers 70 and 72 not only control the deflection units but also control opening and closing of the feed valve. Use of the arrangement of FIG. 5 in the system of FIG. 1 permits a means of drawing on the writing surface 12 a curve having an ordinate axis representing the input from transducer 70 and an abscissa axis representing the input from transducer 72. Thus the curve may represent a plot of temperature vs. pressure, flow rate vs. volume, etc.

FIG. 6 relates to a further refinement of the invention. This modification is designed to meter the volume of ink in the jet stream so as to assure a constant deposit of ink per unit length of travel along the Writing surface of the target. Accordingly, it requires the use of a proportional feed valve 88 instead of an on-off feed valve as used in the embodiment of FIG. 3. In this modification the horizontal (x) and vertical (y) deflection signals are passed to the horizontal and vertical deflection units and also to separate differentiators 90 and 92 in order to achieve output signals varying according to the functions dx/dt and dy/dt, where t represents time. These output signals are then fed to separate squaring amplifiers 94 and 96. Thereafter the squared signals are passed to a summing amplifier 98 where they are added together. The output of summing amplifier 98 in turn is passed to a square root generator 100 to obtain a signal varying according to the function This latter signal is employed to control the proportioning feed valve 88.

Under certain conditions it may be desirable for the stream of ink directed at the writing surface to consist of very fine droplets. This is achieved by using a gas such as air, argon, nitrogen, etc. to atomize the ink. By way of example, atomizing gas fed from a suitable source of supply 106 may be injected via a line 108 into the delivery tube 4 as shown in dotted lines in FIG. 1, or else may be injected into the nozzle 6. The flow of atomizing gas may be continuous or may be controlled by an on-off flow valve 110 that is operated by the same electrical signal employed to control feed valve 8.

It also is possible to control the flow of ink so that it is released in a series of small single droplets. With reference to FIG. 6, this may be achieved by replacing the proportioning feed valve 88 with an on-off feed valve adapted to open and close at a variable controlled frequency, and feeding the output of square root generator to a conventional amplitude-to-frequency-converter 112. The output of the latter is then used to control the on-oif valve feed valve. With this arrangement the feed valve will release one drop for every cycle of the output of converter 112, while at the same time assuring a constant deposit of ink per unit length of travel along the Writing surface. With or without this modification the system of FIG. 6 will provide a constant deposit of ink regardless of the response time, i.e., the time required to deflect the stream.

In the foregoing embodiments of the invention the type of ferrofluid employed is not critical but is a matter of choice depending upon the performance desired. In certain cases, e.g., using ferrofluids of the type described by Rosensweig, the magnetic constituent of the ferrofluid may have sufficient opacity to avoid the need for adding a pigment or dye. However, in other cases a pigment may be required. The pigment may be any of the usual types employed in printing inks, e.g., carbon black, and should have a particle size small enough to remain in suspension in the ferrofluid. The carrier fluid employed should be sufiiciently volatile to evaporate within a few seconds after contacting the printing surface. By way of example but not limitation, the ink may consist of sub-micron size ferrite particles and micron size carbon black suspended in toluene. The amount of carbon black may be varied according to what density of writing is desired. Other carrier fluids that may be used in place of toluene are kerosene, heptane, benzene xylene and methylethylketone.

It is believed to be apparent that the present invention offers a number of advantages. For one thing it is flexible and may be arranged to print on stationary or moving printing surfaces. Except for the special pole pieces employed in the deflection units, the components which it requires are conventional and readily available and the techniques involved in deriving, applying and processing the input data signals are Well within the skill of the art and are generally similar to those employed in conventional data processing and display equipment, notably television. Furthermore the invention has utility in a number of fields where graphic displays are required. For example, it may be used as an xy plotter, as an output display in conjunction with process control and telemetering systems, and as an integral part of a facsimile transmission and reproduction system. The written record produced on the target is permanent and is easy to read. The ink may be formulated so as to have any one of a number of colors, depending upon the pigment or dye employed therein. Further, by way of example, if the writing surface is dark, a white pigment such as titanium dioxide may be used to provide a white ink. It is also possible to incorporate in the ink particulate material of a phosphorescent or luminescent nature. Other advantages and modifications of this invention are believed to be obvious to persons skilled in the art.

While the fundamental novel features of this invention have been shown and described or pointed out, it will be understood that various omissions, substitutions and charges in the construction and mode of operation of the embodiments herein disclosed may be made by those skilled in the art without departing from the spirit of the invention.

What is claimed is:

1. A method of writing in accordance with an electrical information signal comprising removing a ferrofluid ink from a source of supply thereof and directing it as a stream at a target having a writing surface, generating a gradient magnetic field in the path of said stream, and deflecting said stream as it moves toward said target by varying the strength of said gradient magnetic field in response to and in accordance with variations in an electrical information signal.

2. A method according to claim 1 wherein said magnetic field has a gradient transverse to the direction of movement of said stream.

3. A method according to claim 1 further including generating a second gradient magnetic field in the path of said stream that is displaced approximately 90 from the gradient of the other field, and further deflecting said stream by varying the strength of said second gradient magnetic field in response to variations in another electrical information signal.

4. A method of writing comprising removing a ferrofluid ink from a source of supply thereof by means of a conduit and directing it as a stream at a target having a writing surface, deflecting said stream as it moves toward said target by subjecting it to a gradient magnetic field, and repeatedly interrupting flow of ink through said conduit from said source of supply at a frequency such that said stream consists of a series of small droplets of ink.

5. A system for writing comprising a source of supply of an ink having a magnetic polarizability that is substantially uniform, a target with a writing surface, means for removing ink from said source of supply as a stream and directing said streams so that it impinges on said writing surface, and electromagnetic means interposed between said target and said source of supply for magnetically deflecting said stream so as to produce a writing on said surface in response to an applied electrical deflection signal, said electromagnetic means comprising a single pair of spaced pole pieces positioned so that said stream passes between them, a winding on said pole pieces, and means for applying an electrical deflection signal tosaid winding so as to energize said winding and thereby produce a magnetic field with a strength that varies in response to changes in said signal, said pole pieces being shaped and disposed so that said magnetic field has a distinct magnetic gradient in the path of said stream.

6. A system according to claim 5 wherein said electromagnetic means is oriented so as to cause deflection along an axis extending parallel to said writing surface, and each of said pole pieces is elongate in a direction extending at substantially a right angle to said axis and said writing surface.

7. A system according to claim 5 wherein said electromagnetic means is arranged so as to magnetically deflect said stream along a first axis extending parallel to said writing surface, and further including second like electromagnetic means adapted to generate another gradient magnetic field in the path of said stream, said second like electromagnetic means disposed so that said another gradient magnetic field will magnetically deflect said stream along a second axis extending parallel to said writing surface but displaced angularly from said first axis.

8. An oscillographic writing system employing a ferrofluid ink to rovide a written record of electrical information signals comprising a target with a writing surface, a source of supply of a ferrofluid ink, means connected to said source of supply for directing a stream of ferrofluid ink through space at said target so that it impinges on said writing surface, first and second electromagnetic means for magnetically deflecting said stream with first and second magnetic fields so as to produce a Writing on said surface, said first and second electromagnetic means each comprising a single pair of spaced pole pieces with a winding on said pole pieces adapted to be energized by an electrical signal and thereby produce a magnetic field with a strength that varies in response to changes in said signal, said pole pieces being shaped and arranged so that the magnetic fields produced by energizing said windings have a distinct magnetic gradient in the path of said stream with the gradient of one field displaced approximately from the gradient of the other field, and means for providing first and second electrical information signals and applying said first and second information signals to the windings of said first and second electromagnetic means respectively.

9. A system for writing comprising a source of supply of a ferrofluid ink, a target with a writing surface, first means for removing ink from said source of supply as a stream and directing said stream so that it impinges on said writing surface, said first means including a feed valve adapted to open and close at a frequency varying in accordance with the frequency of an electrical control signal applied thereto, second electromagnetic means interposed between said target and said source of supply for magnetically deflecting said stream so as to produce a writing on said surface, said second electromagnetic means being responsive to an electrical deflection signal, third means for applying an electrical deflection signal to said second electromagnetic means, and fourth means for deriving said electrical control signal from said electrical deflection signal so that its frequency is a function of the amplitude of said electrical deflection signal.

10. A system for writing comprising a source of supply of an ink having a magnetic polarizability that is substantially uniform, a target with a writing surface, means for removing ink from said source of supply as a stream and directing said stream so that it impinges on said writing surface, and electromagnetic means interposed between said target and said source of supply for magnetically deflecting said stream so as to produce a writing on said surface in response to an applied electrical deflection signal, said electromagnetic means comprising a pair of spaced pole pieces shaped and disposed so that said magnetic field has a distinct magnetic gradient in the path of said stream, a winding on said pole pieces, and means for applying an electrical deflection signal to said winding so as to energize said winding and thereby produce a magnetlc field with a strength that varies in response to changes in said signal, one of said pole pieces having an end face with a rib and the other pole piece having an end face with a curved recess, said end faces confronting each other with said rib and recess extending substantially parallel to the general path travelled by said stream when it is not deflected by said field.

11. A system for writing comprising a source of supply of an ink having a magnetic polarizability that is substantially uniform, a target with a writing surface, means for removing ink from said source of supply as a streamand directing said stream so that it impinges on said writing surface, said ink-removing means including a nozzle and a proportioning feed valve connected between said nozzle and said source of supply for varying the volume of ink in said stream, electromagnetic means interposed between said target and said source of supply for magnetically deflecting said stream so as to produce a writing on said surface in response to an applied electrical deflection signal, said electromagnetic means comprising a spaced pole pieces positioned so that said stream passes between them, a winding on said pole pieces, and means for applying an electrical deflection signal to said winding so as to energize said winding and thereby produce a magnetic field with a strength that varies in response to pair of changes in said signal, said pole pieces being shaped and disposed so that said magnetic field has a distinct magnetic gradient in the path of said stream, and means for deriving from said deflection signal another signal for controlling said proportioning feed valve so as to provide a constant deposit of ink per unit length along said writing surface. v

12. An oscillographic writing system employing a ferrofiuid ink to provide a written record of electrical information signals comprising a target with a writing surface, a source of supply of ferrofluid ink, ink-feeding means connected to said source of supply for directing a stream of said ink through space at said target so that it impinges on said writing surface, said ink-feeding means including a proportioning feed valve for varying the volume of ink in said stream and control means for controlling said proportioning feed valve so as to provide a constant deposit of ink per unit travel along said writing surface, first and second electromagnetic means for magnetically deflecting said stream with first and second magnetic fields so as to produce a Writing on said surface, said first and second electromagnetic means each comprising a-single pair of spaced pole pieces with a winding on said pole pieces adapted to be energized by an electrical signal and thereby produce a magnetic field with a strength that varies in response to changes in said signal, said pole pieces being shaped and arranged so that the magnetic fields produced by energizing said windings have a distinct magnetic gradient in the path of said stream with the gradient of one field displaced approximately from the gradient of the other field, and means for providing first and second electrical information signals and applying said first and second information signals to the windings of said first and second electromagnetic means respectively, said control means comprising means for differentiating said first and second electrical information signals, means for squaring each of said differentiated signals, means for summing the squared signals, means for deriving a square root signal from the sum of said squared signals, and means for applying said square root signal to said proportioning feed valve.

References Cited UNITED STATES PATENTS 1,841,452 1/1932 Ranger 346-75 X 1,882,043 10/1932 Schroter 178-88 3,060,429 10/ 1962 Winston 346-1 3,169,821 2/1965 Miller 346- 3,287,734 11/1966 Kazan 346-1 FOREIGN PATENTS 821,674 1l/1951 Germany.

JOSEPH W. HARTARY, Primary Examiner US. Cl. X.R. 346-75 

